U.S. Pat. No. 3,455,148 describes a vibration measuring device for piston-type machines or other mechanical machinery in which a piezoelectric crystal is used as an electromechanical transducer for detecting and indicating inadmissibly high vibrations. This is done by evaluating the vibration-connected acceleration and using the evaluation in the structure of the machinery.
An object of the piezoelectric crystal and of the following evaluation circuit is to generate an electrical signal when a predetermined vibration threshold and an acceleration related thereto is exceeded, which is suitable as an indication or for shutting the machinery. High-frequency vibrations of very short duration should not lead to any such indication or disconnection, since they have no undue influence on the machinery. For this purpose, an additional evaluation circuit is provided. Further, adjustable feedback is provided which serves to bring the threshold of a differential amplifier to the permissible vibration limit, depending on the type of the machine.
Piezoelectric elements for use as vibration or acceleration detectors are also employed in the safety engineering of cars where safety devices are actuated by electronic sensors to protect the passengers. This is done when a car strikes against an obstacle, for example, and the actuation may be the release of air bags or the tightening of belts.
The technical problems resulting from the application of these sensors in cars are much greater than their use in stationary machinery. On the one hand, the sensitivity of the sensors must be very high because of the rapidity of the movements. Thus, for example, in an impact at 50 kmh, both the recording of the impact and the safety measures taken, like the tightening of a belt or inflation of the air bag, must be completed within 40 milliseconds (ms). On the other hand, the action must not occur during ordinary conditions for obvious reasons, even though impact-type criteria are provided on the sensor. For example, when a car drives over an obstacle or even in normal braking, the safety system must not be released. It must also be kept in mind that below a certain acceleration, e.g. at about 4 g, the passengers can absorb accelerations even without additional restraining measures. Under these circumstances, the release of the safety devices could even increase the safety risk, so that the opposite of the actual objective would be achieved.
It was found that a crash situation is characterized particularly in that the polarity or sign of the acceleration does not change over a much longer period of time than occurs in intensive short-time accelerations, which are typical when driving over a curb or under a hammer blow. In the latter instances, the polarity of the acceleration changes repeatedly in a very short period of time.
Starting from this background, a piezoelectric impact sensor is known from U.S. Pat. No. 3,701,903 which has an amplitude limiter, an integrator, and a nominal value circuit, in addition to two piezoelectric crystals for shocks from different directions from the front of the system. The amplitude limiter in the form of a Zener diode has the function of limiting the upward amplitude of shock, so that the following integrator attains the threshold voltage only upon the occurrence of shocks with longer lasting accelerations. If the amplitude of shocks is too small, that is, below the level determined by the Zener diode, no output signal is provided from the integrator. The integrator also has the effect that the release voltage for the threshold value switch is attained only after a predetermined period of time. The resetting of the integrator, after the occurrence of shocks below the release threshold, is effected by way of a discharge resistance. As a voltage source for the release of the safety device, a capacitor is used which is charged from a voltage source in the car. Due to the above described arrangement of the piezoelectric crystals, the system reacts in the predetermined manner only to shocks from the front.
Another embodiment of an impact sensor for cars is known from DOS No. 2,207,831. This sensor too, works on the premise that a true release situation is characterized by the unchanged polarity of the voltage tapped from the piezoelectric crystal over a longer period of time. An electronic trigger circuit is arranged behind the piezoelectric element which changes state after attaining a predetermined threshold value. Accordingly, a delay stage is driven which in turn effects the ignition of a release element of the safety device at a preselected time after the trigger circuit has changed state. Preceding the trigger circuit is a potentiometer whose setting represents the change-of-state value of the trigger. The settings of the circuit can be so selected that the trigger stage acts an an integration amplifier. In this manner, a signal proportional to the voltage on the piezoelectric element is generated, so that the delay stage is started and stopped corresponding to this signal.
It follows from the foregoing considerations that sensors of the above described type are based on the evaluation of the duration and amplitude of an occurring acceleration. It is necessary for the reliability of these circuits that measures be taken for the logical processing of the signal in the sensor so the safety device is only released in an actual crash situation. It was found that the reliable operation of the sensor depends to a great extent on the observation of the accompanying circumstances occurring before a possible crash. A number of critical situations can be listed which cannot be processed by the presently known sensors or can only be insufficiently processed.
A critical situation exists, for example, in the collision-type accident. It happens frequently that a driver's own braking efforts would be sufficient for stopping his car, but his car is then hit by a following car. If no special measures are provided within the circuit, this negative acceleration applied to the piezoelectric crystal can reduce the release threshold so much that the sensor will not release during the impact which typically follows. Furthermore, there has not been sufficient consideration given to the fact that the delay time for the release of the safety system depends on the size of the occurring acceleration. That is, the critical speed relative to the passenger is achieved correspondingly sooner at higher accelerations, and, therefore, the safety system must be sooner released than at low accelerations. Consequently, measures must be taken to ensure that the release provided by the sensor will take place when the critical speed is achieved.
Another critical point is the voltage supply for the sensor from the supply system of the car. When starting the car, turning on loads such as lamps, radio, sliding roof, etc., when disconnecting the battery, and in a number of other processes in the car, undefinable conditions appear in the voltage supply system which can lead to misfunctions both during the ride and during the stopping of the car. Finally, since short circuits at any point within the circuit may lead to release of the system, particular attention must be given to making the sensor system of the circuit both reliable and passive. To this end, it is not sufficient merely to disconnect the voltage sources.